Alloy materials including Si and Sn are drawing attention as electrode materials that can be alloyed with Li for lithium secondary batteries. Batteries made with such materials offer high charge and discharge capacity. Among the alloy materials, amorphous alloy materials or microcrystalline alloy materials show superior cycle characteristics to crystalline alloy materials. For example, Japanese patent laid-open No. 2002-216746 discloses a method for forming an amorphous or microcrystalline alloy material comprising Si by adding an element such as Al to Si to prepare molten alloy and then cooling the molten alloy rapidly to thereby obtain a rapidly solidified alloy.
In the preparation of a rapidly solidified alloy containing Si, single phase Si, which has a relatively large crystal particle, should be prevented from being deposited in order to obtain an amorphous or microcrystalline alloy. For this, the content of Si should be no more than 50%. If the Si content is maintained at more than 50%, the amount of a negative active material is decreased and thus the charge and discharge capacity is degraded.
Another method of forming a 100% Si amorphous or microcrystalline structure is to form a Si layer with a Chemical Vapor Deposition (CVD) method by using silane gas. However, this also has shortcomings in that it is difficult to form a sufficiently thick Si layer.